A light-emitting diode (LED) is a semiconductor diode that emits light when driven with a forward biased current. An LED can be used as a regular light source, and has many advantages when compared with conventional incandescent light bulbs or fluorescent lamps. Efficient methods of providing lighting using LEDs are known. For example, an efficient LED lighting method is disclosed in U.S. Pat. Pub US2007/0252805(A1) by the same inventor, which is incorporated herein by reference. As disclosed in US2007/0252805(A1), efficient lighting can be achieved by utilizing the properties that (1) an LED can be turned on or turned off in less than a fraction of a millisecond, and (2) the human eye retains images (brightness) for more than 20 milliseconds after cessation of illumination. A method is disclosed in which a physical switch is used to periodically switch a direct current (DC) powered LED between an on state and an off state at frequencies sufficient to accommodate human eye vision retention properties.
US2007/0252805(A1) includes a description of an approach to driving an LED (or an array of LED) in which a relatively short, intense driving interval, for example, 3-6 ms in duration, is alternated with a relatively long substantially non-driven interval, for example, 30-50 ms in duration. A model for human perception of intensity of the light produced by an LED driven in such a manner accounts for the rapid onset of substantially full perceived intensity (“saturation”) during the short driving interval coupled with a relatively slow decay of perceived intensity during the long non-driven interval. Under this model, a perception of intense illumination is achieved with a relatively low duty cycle during which the LED is actually driven.
Various types of circuits may be used to generate a desired driving voltage (or current) to drive an LED (or LED array) according to the general approach described in US2007/0252805(A1). For example, electrical switching elements (e.g., transistors), or mechanical elements (e.g., relays) may be used. In certain applications, use of such active switching elements may be appropriate given commercial or other considerations.
As outlined above, for some applications, there is a need for the circuitry for driving the LED to have a lifetime of the same order as the LED themselves. In certain examples, particularly in those circuits using relatively high driving currents, the inventors have observed relatively short lifetime for active switching elements of the circuits as compared to the long lifetime for the LED themselves.
Although the lighting method disclosed in US2007/0252805(A1) provides efficiency advantages, use of a physical switch to periodically switch an LED between and on state and an off state is expensive due to the required high frequency of switching, which results in a relatively short physical switch component life-span as compared to the life span of an LED, as well as relatively high replacement costs associated with replacing failed physical switch components.
Articles describing advantages of using LED-base illumination over existing conventional light sources can be found in many places including the internet, journals, and news papers. In view of the advantages of using LED as a light source, there are substantial economic benefits to be gained if the energy required to power an LED can be further reduced from the conventional DC-operated LED lighting as described in US2007/0252805(A1).
A method of periodically alternating one or more diode(s) (including LED) between on and off states is needed. Such a method should take advantage of the physical properties of diode(s) and of human visual perception. In addition, such a method should provide a cost effective device having a life time comparable to that of the diode(s).